Applicability to Evaluating the Sustainability of Chemical Industrial Processes

Applicability to Evaluating the Sustainability of Chemical Industrial Processes 

Co-production is still used in many relevant industrial processes. A typical example is the synthesis of phenol via the cumene process, which involves the formation of acetone as by-product. This example is discussed in a more detail later in Chapter 13. Direct oxidation of benzene to phenol using H2O2 is an attractive industrial alternative, but a main motivation is to avoid the formation of acetone, as there is actually excess acetone on the market. The process requires a single step versus three steps in the cumene process, one of which is related to the synthesis of a cumene hydroperoxide intermediate that shows significant problems of safety. There are thus various aspects that make more sustainable the direct synthesis of phenol over the cumene process, but atom economy (or related mass intensity indicators) is not the correct indicator to assess sustainability. 

We may further add that, in the chemical industry, process improvement results in a significant reduction of waste, even if this aspect does not appear, at least in a transparent way, in the cited indicators. 

The use of oxygen instead of air implies that the same partial pressure can be used with a much lower total pressure than with air, thus making it feasible to possibly reduce the total pressure, with obvious energy advantages. Moreover, an increase in the reaction rate makes it possible to reach the same productivity while lowering the reaction temperature, with possible benefits from the selectivity point of view when several products are formed in the reaction. Lower nitrogen contents, a ballast with very poor thermal conduction properties, also allow better control of the temperature profile in the presence of strongly exothermal reactions. 

An interesting example is ethene oxychlorination where the use of pure O2 instead of air as the oxidizing agent improves productivity and reduces the environmental impact. The oxychlorination of ethene to 1,2-dichloroethane (DCE) reaction  

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